A network is a collection of interconnected nodes that exchange information. The network may be configured as a local-area network (“LAN”) or wide-area network, such as the Internet. Each network node may be a computer or any other device that is configured to communicate with other nodes in the network. The network nodes typically communicate with one another by exchanging information in accordance with predetermined network communication protocols. In this context, a protocol is a set of rules defining how information is exchanged between network nodes.
Ethernet is a common network communication protocol used in LANs. The Ethernet protocol is set forth in the publicly-available Institute of Electrical and Electronics Engineers (“IEEE”) Standard 802.3, entitled “Carrier Sense Multiple Access With Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications,” which is hereby incorporated by reference in its entirety. The IEEE Standard 802.3 describes data-packet formatting for creating Ethernet data packets, and further describes different combinations of baseband data rates (i.e., without frequency modulation) and physical media for transmitting Ethernet data packets between network nodes.
As used herein, an “Ethernet mode” corresponds to a particular combination of baseband data rate and physical transmission medium. The IEEE Standard 802.3 describes various Ethernet modes including, for example, 10BASE-T, 100BASE-TX (“Fast Ethernet”), and 1000BASE-T (“Gigabit Ethernet”). More specifically, 10BASE-T supports baseband Ethernet data transmissions up to 10 megabits per second (“Mbps”) over twisted-pair cables. 100BASE-TX supports baseband transmissions up to 100 Mbps over twisted-pair cables; 1000BASE-T supports baseband transmissions up to 1 gigabit per second (1000 Mbps) over twisted-pair cables. While 10BASE-T, 100BASE-TX, and 1000BASE-T are popular Ethernet modes in modern LAN architectures, it will be apparent that other Ethernet modes are possible. Accordingly, the 10BASE-T, 100BASE-TX, and 1000BASE-T Ethernet modes are discussed throughout this disclosure by way of example and not limitation.
In practice, 10BASE-T and 100BASE-TX LAN connections are typically deployed over conventional “Category-5” cables having four pairs of unshielded twisted copper wires. 1000BASE-T connections typically use enhanced Category-5, or “Category-5e,” cables. Both Category-5 and Category-5e cables have 100 ohm impedances and, thus, require 100 ohm terminations to prevent signal reflections. In this disclosure, the terms “Category-5 cable” and “CAT5 cable” generally refer to any cable that exhibits the electrical characteristics of a conventional Category-5 or Category-5e cable.
Typically, a network node comprises a network interface card (“NIC”) adapted to transmit and/or receive data. The NIC may contain hardware and software drivers for transmitting data using a selected Ethernet mode. To that end, the NIC may employ line driver circuitry to transmit and/or receive Ethernet data over a physical transmission medium, such as a Category-5 cable. FIG. 1 illustrates a schematic block diagram of an exemplary Ethernet connection (or “link”) 100 having a transmitter side 110 and a receiver side 140 interconnected by a Category-5 cable 130. The transmitter side comprises line driver circuitry, e.g., located in a first NIC, configured to transmit Ethernet data over the cable 130 to the receiver side 140, e.g., located in a second NIC. The transmitter side 110 is electrically isolated from the cable 130 and the receiver side 140 by a transformer 120 having a one-to-one turns ratio. The transmitter side circuitry also includes a pair of 50 ohm resistors R1 and R2 that are impedance matched with effective 50 ohm resistances R3 and R4 in the Category-5 cable 130.
The exemplary line driver circuitry shown in FIG. 1 transmits Ethernet data as a differential output signal having a positive output voltage txp and a negative output voltage txn. The resulting Ethernet signal is therefore the difference of the positive and negative output voltages, i.e., txp−txn. In 10BASE-T Ethernet mode, the typical positive output voltage txp is greater than 2.2 volts peak-to-peak (Vpp) and the resulting differential output signal is therefore greater than 4.4 Vpp. In contrast, 100BASE-TX and 1000BASE-T modes employ significantly lower-amplitude signals, e.g., having a positive output voltage txp around 1 Vpp and a differential output signal around 2 Vpp. Although the 100BASE-TX and 1000BASE-T output signals have similar peak-to-peak voltage swings, the IEEE Standard 802.3 specifies that the 1000BASE-T output signal, unlike the 100BASE-TX signal, is encoded using five-level pulse-amplitude modulation for better bandwidth utilization.
It is often desirable for a NIC to be capable of transmitting data using more than one Ethernet mode. For example, the NIC may be located in a device that is initially configured to communicate over a 10BASE-T Ethernet link, but subsequently may be connected to a faster 100BASE-TX link. In this example, the line driver circuitry in the NIC must be capable of transmitting both 10BASE-T and 100BASE-TX Ethernet signals. By way of example, FIG. 2 illustrates one possible implementation of a prior art multimode Ethernet line driver circuit that can be configured for either 10BASE-T or 100BASE-TX operations.
As shown in FIG. 2, an exemplary transmitter side 200 comprises an Ethernet line driver circuit including a pair of current sources 210 and 220. The current sources alternatively could be replaced with voltage sources (not shown). The line driver also includes a pair of 50 ohm resistors R1 and R2 that are impedance matched with the Category-5 cable 130. The current sources 210 and 220 output respective currents I1 and I2 for generating a differential Ethernet signal through the transformer 120. The transformer is center-tapped and has its center tap connected to a constant supply voltage Vcc. The supply voltage Vcc therefore sets the common-mode voltage of the Ethernet signal. As used herein, a “common mode voltage” is a constant voltage offset on which an alternating-current (“AC”) signal may be modulated.
To effectively convert between 10BASE-T and 100BASE-TX modes of operation using the multimode line driver of FIG. 2, the current sources 210 and 220 must be able to generate different output signal amplitudes. Specifically, and as noted above, each of the current sources 210 and 220 must generate 2.2 Vpp output signals for 10BASE-T operations, whereas the current sources only need to generate 1 Vpp output signals for 100BASE-TX (or 1000BASE-T) operations.
Although the multimode Ethernet driver shown in FIG. 2 is feasible, it suffers significant disadvantages. Most notably, the power efficiency of the line driver circuit is directly related to the ratio of its output driver voltage swing to its supply voltage. Therefore, since the required voltage swing (e.g., 1 Vpp) in 100BASE-TX mode is much less than the required voltage swing (e.g., 2.2 Vpp) in 10BASE-T mode, using a fixed supply voltage Vcc for both Ethernet modes generally results in poor power efficiency in the 100BASE-TX mode. More generally, in order for the multimode Ethernet line driver of FIG. 2 to achieve useful power efficiencies in the 100BASE-TX or 1000BASE-T modes, the supply voltage Vcc would have to be chosen around 0.5 volts, which is too low for practical implementations.
One known solution for improving power efficiencies in multimode Ethernet line drivers is to use separate output driver circuitry for the 10BASE-T and 100BASE-TX (or 1000BASE-T) modes, so as to increase output driver voltage swings in the 100BASE-TX (or 1000BASE-T) mode. Although the supply voltage Vcc may remain constant for both Ethernet modes, the 10BASE-T mode may use an output driver circuit having a greater voltage swing than the output driver circuitry for the 100BASE-TX (or 1000BASE-T) mode. As a consequence, the ratio of output driver voltage swing to supply voltage is maintained for the 10BASE-T mode and improved for the 100BASE-TX (or 1000BASE-T) mode.
FIG. 3 illustrates an example of a multimode Ethernet line driver circuit having separate output driver circuitry for 10BASE-T and 100BASE-TX signal generation. The exemplary transmitter side 300 includes a 100BASE-TX driver circuit comprising a pair of voltage sources 310 and 320 that respectively generate voltages V1 and V2. The line driver of FIG. 3 also includes a pair of 50 ohm termination resistors connected in series with the voltage sources 310 and 320. In the 100BASE-TX mode, the current sources 330 and 340 are “idle” (i.e., not generating currents) and essentially act as open circuits. Because the termination resistors R1 and R2 are series-connected to the voltage sources 310 and 320, the voltage drops across these resistors enables the output voltages V1 and V2 to increase, e.g., around 2 Vpp, while still being able to generate the requisite 1 Vpp output signals at the transformer 120. As a result, the multimode line driver circuit of FIG. 3 requires only a moderate supply voltage, e.g., equal to 2.5 V, to achieve usable power efficiencies in the 100BASE-TX mode.
Despite having certain advantages for 100BASE-TX operations, the multimode line driver circuit of FIG. 3 suffers significant drawbacks in the 10BASE-T mode. In the 10BASE-T mode, the current sources 330 and 340 generate respective currents I1 and I2. The resistors R1 and R2 become termination resistors for the current sources 330 and 340. In addition, the voltage sources 310 and 320 remain powered on and exhibit low impedances, thereby providing an AC ground potential for the 10BASE-T driver circuit. Because the voltage sources essentially function as short circuits to ground, the resulting 10BASE-T line driver circuit of FIG. 3 functions in the same manner as the line driver circuit shown in FIG. 2.
Problems typically arise in the 10BASE-T circuit configuration of FIG. 3 because the voltage sources 310 and 320 are typically implemented using operational amplifiers that are unable to accommodate the fast current-sinking demands required to maintain the AC ground potential during 10BASE-T data transmissions. Moreover, the voltage sources 310 and 320 continue to consume power as they remain active in providing the AC ground potential and, as a result, reduce the power efficiency of the 10BASE-T driver circuit. Such current-sinking and power-consumption limitations of conventional operational amplifiers complicates the design of the voltage sources 310 and 320 and limits their usefulness in prior multimode Ethernet line drivers.